Conductor sizing for CC fusing

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VM321

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Hello,

Ive got a motor circuit for 480VAC 3Ph 5HP motor.
The fusing for 30A CC time delay for Branch protection (300% of motor FLC). Does the conductor from fuse to motor contactor have to be a minimum of 10AWG because of the fuse size or 125% of the motor load (7.6A) as small as 12-14AWG.
 
It is my opinion that when you install an OCPD lager than what is permitted in Table 430.52, you will need to install conductors that have an ampacity that equals or exceeds the rating of the OCPD. Normally a time delay fuse is limited to 175% of the motor current, however Note 1 to the table that says for Class CC fuses you can use the non-time delay column which is 300%.

The full load current of your motor is 7.6 amps per table 430.250. 300% of that is 22.8 amps. Table 240.6(A) shows that 25 amps is a standard size OCPD, and would be the maximum size Class CC fuse permitted for this circuit.

That brings me back to my first statement, and since the 30 amp is larger than what is permitted by 430.52, you need to use 10AWG copper conductors.
 
Other than by error it does not seem likely that would have that situation (30 amp fuse) as the motor would still require proper GFSC protection.
 
It would depend on whether the fuse is the ONLY protective element in the circuit or if there is an overload relay involved. If you are using the fuse AS the only OL protection for the motor (some people do this), then the fuse cannot be that large. If there is an OL relay or MMS in the circuit, then the fuse is only acting as the Short Circuit protection, the conductors are considered protected by the OL relay, even though it is downstream.
 
don_resqcapt19 said:
It is my opinion that when you install an OCPD lager than what is permitted in Table 430.52, you will need to install conductors that have an ampacity that equals or exceeds the rating of the OCPD. Normally a time delay fuse is limited to 175% of the motor current, however Note 1 to the table that says for Class CC fuses you can use the non-time delay column which is 300%.

The full load current of your motor is 7.6 amps per table 430.250. 300% of that is 22.8 amps. Table 240.6(A) shows that 25 amps is a standard size OCPD, and would be the maximum size Class CC fuse permitted for this circuit.

That brings me back to my first statement, and since the 30 amp is larger than what is permitted by 430.52, you need to use 10AWG copper conductors.
Click to expand...
Yes. I made an error when making an example. Even with 25A fuse typically it would be recommended to use 10AWG. When using a contactor and overload rated for 5Hp Motor will, will a 14AWG or even 12AWG be acceptable?
 
Jraef said:
It would depend on whether the fuse is the ONLY protective element in the circuit or if there is an overload relay involved. If you are using the fuse AS the only OL protection for the motor (some people do this), then the fuse cannot be that large. If there is an OL relay or MMS in the circuit, then the fuse is only acting as the Short Circuit protection, the conductors are considered protected by the OL relay, even though it is downstream.
Click to expand...
Yes, there is OL protection rated for 5HP 480VAC motor.
 
VM321 said:
Yes. I made an error when making an example. Even with 25A fuse typically it would be recommended to use 10AWG. When using a contactor and overload rated for 5Hp Motor will, will a 14AWG or even 12AWG be acceptable?
Click to expand...
Since the 25 amp fuse is permitted to provide the short circuit and ground fault protection for the motor circuit conductors, those conductors only need to have an ampacity of 9.5 amps.
 
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